In optical communication networks, semiconductor optical amplifiers (SOAs) are used. An automatic level control (ALC) operation for such a semiconductor optical amplifier is achieved by monitoring optical power output from the semiconductor optical amplifier and performing feedback control of a driving current for the semiconductor optical amplifier.
A signal light intensity control apparatus has been proposed for monitoring output signals including an amplified spontaneous emission (ASE) signal and controlling the driving current with a feedback configuration including an average value monitor (see, for example, Japanese Laid-open Patent Publication No. 2003-46186).
It is assumed that the control of the driving current based on the relationship between the driving current and ASE is performed with the technique disclosed in Japanese Laid-open Patent Publication No. 2003-46186. In this case, when the relationship between the driving current and ASE is changed, it is necessary to interrupt an input signal and correct the relationship.